Method and apparatus for increasing fuel efficiency in nuclear reactors

ABSTRACT

A method and apparatus for achieving increased fuel efficiency in a nuclear reactor wherein spectral shift is utilized to adjust for excess reactivity. This feature is achieved by stationary displacer rods within the fluid moderator of the reactor, with these stationary displacer rods decreasing in effective volume during operation of the reactor whereby the effective volume of the fluid moderator increases as the nuclear fuel is burned. This decrease in effective volume is achieved by providing a sacrificial material in the displacer rods that is dissolved (or volatilized, etc.) by the fluid moderator. The compositon of the sacrificial material can be varied along the length of the rod so as to achieve the desired reduction of volume despite a temperature gradient along the rod.

DESCRIPTION

1. Technical Field

The present invention relates generally to nuclear reactors, and moreparticularly to a type of nuclear reactor known as a "spectral shift"reactor whereby excess reactivity is provided initially, with some meansof regulating the level of the reactor during reactor operationlifetime. More specifically, the present invention provides a means forincreasing fuel efficiency by automatically changing the relativefuel-to-moderation volumes during the reactor operation.

BACKGROUND ART

In every nuclear reactor there must be arranged a quantity offissionable material as a fuel and other materials as a moderator suchthat a "chain" reaction is achieved. The mass of fissionable material istermed a "critical mass". In order that the nuclear reactor can beoperated over an appreciable period of time there must be included anexcess of fuel above the critical mass, with this excess representingthe fuel that will be consumed during operation of the reactor. As thisextra fuel makes available a quantity of neutrons greater than thequantity necessary to perpetuate a controlled chain reaction, theseexcess neutrons must be absorbed in some manner so that an uncontrolledreaction does not result. The inherent ability of the excess fuel toproduce these excess neutrons is generally referred to as "excessreactivity".

In the field of liquid (usually water) moderated nuclear reactors, suchas pressurized water reactors (PWR), one technique for the control ofreactivity is to produce an initial "spectral shift" which has theeffect of increasing the epithermal (low reactivity) part of the neutronspectrum at the expense of the thermal (high reactivity) part. Thisresults in production of fewer thermal neutrons and decreased fission.Then, as fission decreases during extended reactor operation, a reverseshift back to the thermal part of the neutron spectrum is undertaken.

There have been numerous systems developed to achieve this spectralshift. One such system is described in U.S. Pat. No. 3,081,246 issued toM. C. Edlund on Mar. 12, 1963. This system utilized the control of theratios of heavy and light water used as moderator (and coolant) in therector during operation. More recently various mechanical systems havebeen developed to effect the volumetric ratio between the fuel and themoderator to achieve the spectral shift concept. Typical of thesesystems are described in U.S. Pat. Nos.: 4,657,726 issued to D. B.Lancaster, et al., on Apr. 14, 1987; 5,683,103 issued to R. G. Lott, etal., on July 28, 1987; 4,683,116 issued to H. M. Ferrari, et al., onJuly 28, 1987; 4,687,620 issued to A. J. Impink, Jr., on Aug. 18, 1987;4,687,621 issued to H. M. Ferrari on Aug. 19, 1987; 4,687,627 issued toJ. F. Wilson, et al., on Aug. 18, 1987; 4,710,340 issued to W. J.Dollard, et al., on Dec. 1, 1987; and 4,716,007 issued to W. R. Carlson,et al., on Dec. 29, 1987.

In all but the '621 of the "mechanical regulation" patents, there are aplurality of "displacer rods" that can be moved within the reactor.Initially these displacer rods are fully inserted so as to displace aportion of the water within the reactor. As reactor operation proceeds,these rods are removed so as to add a higher proportion of water andthus more moderation as the fuel is consumed to achieve the spectralshift. Generally these displacer rods are grouped for a single fuelelement, or a group of elements, so that a single mechanism can be usedto accomplish the removal. As such, groups of displacer rods areattached to a "spider", with that spider being moved axially in thereactor with a suitable drive means (usually a motor-gear means). Inorder that this removal can be effectively achieved, each displacer rodmust be provided with guides to prevent non-axial movement. For a givenreactor, many groups of displacer rods are used, and it may be desirablethat removal of one group is at different times relative to anothergroup. This removal must be accomplished without deleteriously affectingtemperature and neutron flux gradients within the reactor. Thus, verycomplex mechanical means and controls are required to accomplishregulation of appropriate moderation of the nuclear reaction with thedisplacer rods of the prior art.

In the '621 patent, these displacer rods contain burnable neutron poisonmaterial. Provision is made, via rupture elements, to permit gradualdissolution of the burnable poison material, with this material enteringinto the coolant and thus the moderator. This poison provides control ofthe excess reactivity. As the poison burns, together with the burn-up ofthe fuel, the reactor continues to be controlled.

Of course, in any of the reactor designs, there are normal control rodsthat regulate the level of operation of the nuclear reactor.

Accordingly, it is an object of the present invention to provide amethod and apparatus for increasing the fuel efficiency of a nuclearreactor without adding elaborate mechanical and electrical controls.

It is another object of the present invention to provide this increasedfuel efficiency using substantially conventional reactor constructionwithout the complexity of movement of displacer rods as called for inthe prior art reactor designs.

A further object of the present invention is to provide displacer rodsfor a nuclear reactor to initially provide proper moderation for anyexcess reactivity of a pressurized water reactor, with these displacerrods having a selected dissolution or volatilization/sublimation rate,whereby the volume of the displacer rods is gradually decreased as thenuclear fuel is burned so as to control (increase) the volume of themoderator during reactor operation.

Still another object of the present invention is to provide a spectralshift nuclear reactor in which displacer rods formed of a sacrificialmaterial provide for a change in the fuel-to-moderator ratio withoutmechanical movement and take into account temperature and neutron fluxgradients within the reactor.

Another object of the present invention is to provide controlleddissolution of displacer rods in a pressurized water reactor, with theproduct of that dissolution having no effect upon the nuclearcharacteristics of the reactor.

These and other objects of the present invention will become apparentupon a consideration of the drawings that follow together with adetailed description of the invention.

DISCLOSURE OF THE INVENTION

In accordance with the present invention, a nuclear reactor is providedthat contains displacer rods throughout the reactor so as to provide forthe control of excess reactivity as the reactor is operated. Thesedisplacer rods, which appropriately displace a controlled portion offluid moderator within the reactor, are fabricated from a sacrificialmaterial that slowly dissolves, volatilizes or sublimes during operationsuch that the effective quantity of the fluid moderator increases duringthe life of the reactor as the fuel material is burned. The displacerrods are fabricated from a material that does not poison the reactor orhave any other nuclear effect upon the nuclear reaction. The choice of amaterial for the displacer rods depends upon the rate of decrease of thedisplacer rod volume in the specific fluid moderator material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing illustrating a partial vertical cross section oftypical fuel element of a pressurized water nuclear reactor showing theposition of displacer rods located as interspersed within fuel elementsof the reactor.

FIG. 2 is a drawing illustrating a partial transverse cross section ofthe fuel element of FIG. 1 further showing the position of the displacerrods.

FIG. 3A is a drawing illustrating a portion of one embodiment of adisplacer rod designed to accomplish the objects of the presentinvention, with the sacrificial material illustrated at the beginning ofoperation of the nuclear reactor in which it is utilized.

FIG. 3B illustrates the displacer rod of FIG. 3A after a period ofoperation within the nuclear reactor.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be best understood by reference to FIGS. 1and 2 which are, respectively, vertical and horizontal cross sections ofportions of a pressurized water reactor utilizing displacer rods of thepresent invention. The overall construction of such nuclear reactorswill be known to those skilled in the art. Also, the generalconstruction of these reactors is given in numerous of the above-citedpatents, for example U.S. Pat. No. 4,716,006 that is incorporated hereinby reference to teach the overall construction of pressurized waterreactors.

FIG. 1 is a cut-away drawing of a single fuel element 10 of such areactor. It contains, for example, a plurality of elongated metal-cladfuel rods 12 held in suitable upper and lower grids 14, 16. The fuelelement 10 also has appropriately spaced guide tubes 18 to be used forregular axially movable control rods (not shown). A portion of theseguide tubes 18 is used to support displacer rods 20 to be discussed inconnection with FIGS. 3A and 3B. It will be understood that a givennuclear reactor will have a plurality of these fuel elements 10, andthat the number of fuel rods 12, their spacing and their fuel loadingmay vary depending upon the position of that fuel element in a specificnuclear reactor. Water is caused to flow through the interstices betweenthe fuel rods 12 and the displacer rods 20, with this water (in thistype of reactor) providing both the moderation of the neutrons and thecooling of the fuel elements.

A transverse cross section of a portion of a typical nuclear fuelelement of FIG. 1 showing the positional relationship of the fuel rods12 and the displacer rods 20 is shown in FIG. 2. Also indicated is atypical instrument thimble 21 for this fuel element. It can be seen fromthese figures that the displacer rods 20 exclude the water from aportion of the fuel element 10. As discussed above, this is required toovercome the excess reactivity of the nuclear reactor. In most of theprior art, these displacer rods are completely withdrawn at selectedpositions and times so as to increase the quantity of the moderatorwithin the fuel elements. According to the present invention, however,the displacer rods 20 are not moved.

Referring now to FIGS. 3A and 3B, one embodiment of a displacer rod 20is illustrated that does not require any movement in order to regulatethe excess reactivity of the reactor. The various components thereof areillustrated as enlarged in order to better understand the construction.In this particular embodiment, there is a support sleeve 22 which isperforated as at 24 such that the fluid moderator of the reactor canpenetrate into the interior of the sleeve. Within the sleeve 22 therecan be axially-spaced support plates 26 which, in turn, can also beprovided with perforations as at 28. Supported by these plates 26 areelements 30 of a material that has a selected sacrificial (dissolution,volatilization, sublimation, etc.) rate in the fluid moderator such thatthe volume of the elements 30 decreases at a rate to increase theproportion of the fluid moderator within the reactor at a ratesufficient to compensate for the changing excess reactivity of thereactor. In a reactor having water as the moderator, the elements 30 canbe fabricated from one of the slowly-dissolving aluminum alloys, forexample. As will be understood from a table referenced hereinafter,several materials are available with slow dissolution rates that can beused for this purpose. Since there are vertical temperature and neutronflux gradients in this type of reactor, and since the change in excessreactivity is non-linear, different alloys can be used along the lengthof a given displacer rod 20 to achieve the desired dissolution rate atthe specific temperature and flux of those locations and thereby providethe desired rate of change of the volume of the displacer rods tocorrect for temperature and radiation effects.

Other methods for control of the rate of displacer rod volume changerelate to the construction of the displacer rod (such as changes in thecomponents thereof described with respect to FIGS. 3A and 3B, below).This control of dissolution, volatilization, sublimation, etc. can beeffected by controlling the surface-to-volume ratio of any sacrificialcomponent of the displacer rods. For example, various shapeconfigurations are envisioned, such as providing passageways through agenerally cylindrical body. Other methods of effecting a change in thesurface-to-volume ratio will be known to persons skilled in the art.

In FIG. 3A, the elements 30 of sacrificial material are depicted aspellets for convenience of illustration. Typically, these pellets wouldhave a diameter of about 0.8 in. In other embodiments they can beconfigured as spherical or multiple spheres that provide further controlof the change in the relative surface areas and the compositions of theunits. Thus, the present invention is not to be limited by the physicalconfiguration of the "sacrificial" material within the displacer rods 20or by the physical arrangement or presence of the containment sleeves22.

FIG. 3B illustrates the general structure of the displacement rods aftersubstantial operation of a nuclear reactor into which these rods havebeen inserted. This shows that a portion of each of the elements 30' ofsacrificial material has been removed; however, there is substantialstructural integrity to the elements so that fragments are notincorporated into the flowing coolant/moderator.

Not all reactors will require that the sacrificial material be containedin a sleeve. It may be in the form of rods or a series of unclad pelletswith, for example, some form of stiffener to prevent entrapment ofmaterial in the flowing coolant. The particular choice of structure willdepend upon the particular reactor environment. The desiredcompositional change along the length of the rods can be achieved byconventional powder-metallurgy techniques, for example.

Although the structure illustrated in FIG. 3A is initially designed foruse in a pressurized water reactor where the moderator is the flowingwater coolant and the sacrificial material dissolves in the water, thesame principal can be applied to nuclear reactors where it is desired togradually change (increase) the ratio of a fluid (liquid or gas)moderator with respect to the quantity of unburned nuclear fuel. Thisfuel moderator can be either a liquid or a gas. Typical of suchmoderators are liquid sodium or helium. However, the concept of using asacrificial material is not limited to these named moderators, but isapplicable to all fluid moderators. In these instances, the material ofthe sacrificial material is chosen to provide a desired rate ofdissolution, volatilization or sublimation in the fluid of choice.

It is preferred that the reaction at the displacer rod result in no finematerials. For example, in a liquid-cooled reactor it is desired thatthe sacrificial material result in an ionic form within the fluid. Sincesome filtering of the fluid moderator can be accomplished in normaloperation of the reactor and since most reactors provide for a dailyfiltration, an ion exchange bed can be added to remove the results ofthe dissolution. In the case of gas-cooled reactors, it is desired thatthe sacrificial material be volatilized or sublimed. In this case,appropriate gaseous separation techniques would be applied.

For water cooled and moderated nuclear reactors an average corrosionrate of about 1.3 mg/cm² -day will be required, although a range ofabout 0.5 to about 2.5 mg/cm² -day is envisioned for the various typesof nuclear reactors and for the non-linear change in the excessreactivity. This assumes a normal operating life of the reactor at 18 to24 months. The actual rate will depend upon the temperature and pressureof the system. For a PWR operating at 580 degrees F. and a pressure of2250 psi, the average corrosion rate to achieve the necessary change involume will be about 1 mg/cm² -day.

Listed in the single Table are several aluminum alloys with thepublished corrosion (dissolution) rate in water. It can be seen thatthere are numerous of these alloys that will provide the rates for thereactors currently of interest. Thus, knowing the operating temperatureand pressure, the excess reactivity that is to be controlled, and thecorrosion rates in the fluid of the reactor, an improved displacer rodcan be constructed that will automatically adjust in volume as thenuclear fuel is burned. Of course, persons knowledgeable in corrosion,volatilization and sublimation are aware of other materials that exhibitrates in this range. In this manner, the fuel efficiency of the reactorwill be increased in a much less costly manner than taught by the priorart.

Although certain constructions and materials are discussed herein forillustration, these are not given as a limitation of the presentinvention. Rather, the invention is to be limited only by the appendedclaims or their equivalents when read together with a detaileddiscussion of the invention.

                  TABLE                                                           ______________________________________                                        CORROSION RATES OF ALUMINUM                                                   ALLOYS IN AQUEOUS SOLUTIONS                                                   TREATMENT                                                                     TEST SOL.            RATE                                                     ALLOY   TEMP.    pH    Other   mg/cm.sup.2 -day                                                                        REF.                                 ______________________________________                                        1100 H14                       0.255     1                                    3004 H34                       0.306     1                                    4043 H14                       0.248     1                                    5005 H34                       0.276     1                                    5050 H34                       0.258     1                                    5052 H34                       0.268     1                                    5154 H34                       0.241     1                                    5454 O                         0.257     1                                    5454 H34                       0.253     1                                    5456 O                         0.282     1                                    5083 O                         0.347     1                                    5083 H34                       0.277     1                                    5086 H34                       0.322     1                                    2014 T6                        0.477     1                                    2024 T3                        0.756     1                                    2024 T86                       0.596     1                                    2024 TB1                       0.536     1                                    6061 T4                        0.028     1                                    6061 T6                        0.312     1                                    7075 T6                        0.509     1                                    7079 T6                        0.469     1                                    1199                           1.15      2                                    5154 H38                       1.04      2                                    5454 H34                       1.11      2                                    5457 H34                       1.05      2                                    5456 O                         2.18      2                                    5456 H321                      0.12      2                                    5083 O                         1.11      2                                    5086 O                         1.07      2                                    M388    500      5             22.4      3                                    M388    500      6.7           28        3                                    M388    422            300 PSIG                                                                              9.35      3                                    X8001   500      5.5           92.3      4                                    X8001   600      5.5           240       4                                    X2219   550      9             100       4                                    198X    600      5.5           33        4                                    ______________________________________                                         REFERENCES:                                                                   1. ASM, "METALS HANDBOOK", Ninth Ed, Vol 13, pp 599. Weathering data for      1.27 mm thick Al alloys after 7 years exposure                                2. ASM, "METALS HANDBOOK", Ninth Ed, Vol 13, pp 605. Summary of data from     10 years seawater exposures                                                   3. C. R. BREDEN, N. R. GRANT, "SUMMARY OF CORROSION INVESTIGATIONS ON HIG     TEMPERATURE ALUMINUM ALLOYS", Argonne National Laboratory, February 1960.     Flow of water (7 fps)                                                         4. N. R. GRANT, "SUMMARY OF CORROSION INVESTIGATIONS OF HIGHTEMPERATURE       ALUMINUM ALLOYS", Argonne National Laboratory, September 1961. Flow of        water (7 fps)                                                                 5. ASM ENGINEERING BOOKSHELF, "SOURCE BOOK ON SELECTION AND FABRICATION O     ALUMINUM ALLOYS", American Society for Metals, 1978, pp. 9-11.           

We claim:
 1. An improved method of producing a spectral shift in anuclear reactor to achieve increased nuclear fuel efficiency, saidnuclear reactor containing a fluid moderator juxtaposed with fuelelements containing said nuclear fuel, which comprises disposing withinsaid fluid moderator stationary non-poison displacer rods for achievingsaid spectral shift, said displacer rods exhibiting a continuousreduction in volume during operation of said nuclear reactor wherebysaid fluid moderator increases in volume as said nuclear fuel is burnedin said nuclear reactor.
 2. The method of claim 1 wherein each of saiddisplacer rods is fabricated from a material having a compositionvarying along a length of said displacer rod whereby said continuousreduction in volume is achieved according to a temperature gradientalong said displacer rod in said nuclear reactor.
 3. The method of claim1 wherein said displacer rods are reduced in volume by continuousdissolution in said fluid moderator.
 4. The method of claim 1 whereinsaid displacer rods are reduced in volume by continuous volatilizationin said fluid moderator.
 5. The method of claim 3 further comprising atleast periodically processing said moderator for removal of products ofsaid dissolution.
 6. The method of claim 4 further comprising at leastperiodically processing said moderator for removal of products of saidvolatilization.
 7. The method of claim 1 wherein said displacer rods arefabricated of an inert sleeve containing a non-poison sacrificialmaterial having a dissolution rate to provide said continuous increaseof volume of said moderator.
 8. An improved method of producing aspectral shift in a pressurized water nuclear reactor to achieveincreased nuclear fuel efficiency, said nuclear reactor containing awater moderator-coolant juxtaposed with fuel elements containing saidnuclear fuel, which comprises disposing within said watermoderator-coolant, and interposed with said fuel elements, stationarynon-poison displacer rods for achieving said spectral shift, saiddisplacer rods exhibiting a continuous reduction in volume duringoperation of said nuclear reactor whereby said water moderator-coolantincreases in volume as said nuclear fuel is burned in said nuclearreactor.
 9. The method of claim 8 wherein each of said displacer rods isfabricated from a material having a composition varying along a lengthof said displacer rod whereby said continuous reduction in volume isachieved according to a temperature gradient along said displacer rod insaid nuclear reactor.
 10. The method of claim 8 wherein said displacerrods are reduced in volume by continuous dissolution in said watermoderator-coolant.
 11. The method of claim 8 wherein said displacer rodsare fabricated of an inert sleeve containing a non-poison sacrificialmaterial having a dissolution rate of about 0.5 to about 2.5 mg/cm² -dayto provide said continuous increase of volume of said watermoderator-coolant.